Cellulosic fibrous structures, such as paper, are well known in the art. Frequently, it is desirable to have regions of different basis weights within the same cellulosic fibrous product. The two regions, as exhibited by paper in the prior art, serve different purposes. The regions of higher basis weight impart tensile strength to the fibrous structure. The regions of lower basis weight may be utilized for economizing raw materials, particularly the fibers used in the papermaking process and to impart absorbency to the fibrous structure. In a degenerate case, the low basis weight regions may represent apertures or holes in the fibrous structure. However, it is not necessary that the low basis weight regions be apertured.
The properties of absorbency and strength, and further the property of softness, become important when the fibrous structure is used for its intended purpose. Particularly, the fibrous structure described herein may be used for facial tissues, toilet tissue, and a paper towel, each of which is in frequent use today. If these products are to perform their intended tasks and find wide acceptance, the products must exhibit and maximize the physical properties discussed above. Tensile strength is the ability of a fibrous structure to retain its physical integrity during use. Absorbency is the property of the fibrous structure which allows it to retain contacted fluids. Both the absolute quantity of fluid and the rate at which the fibrous structure will absorb such fluid must be considered when evaluating one of the aforementioned consumer products. Further, such paper products have been used in disposable absorbent articles such as sanitary napkins and diapers.
Several attempts have been made in the art to provide efficient and economical means to manufacture paper having two different basis weights. One of the very early attempts is illustrated in Motz U.S. Pat. No. 795,719 issued Jul. 25, 1905, which patent discloses a Fourdrinier wire having a number of upstanding protuberances and which is passed between two rollers. One advance over Motz is illustrated by Griswold U.S. Pat. No. 3,025,585 issued Mar. 20, 1962 which discloses a belt having tapered projections 61 that rearrange fibers deposited thereon.
Various shapes of protuberances have been used in conjunction with papermaking machines, yielding differing basis weight regions, such as low basis weight regions of varying shapes. For example, Greiner et al. U.S. Pat. No. 3,034,180 issued May 15, 1962 discloses protuberances which are pyramid shaped, cross-shaped, etc. Even the knuckles of a Fourdrinier wire may be utilized as upstanding protuberances, as illustrated in Heller et al. U.S. Pat. No. 3,159,530 issued Dec. 1, 1964.
Instead of apertures, Benz U.S. Pat. No. 3,549,742 issued Dec. 22, 1970 shows a foraminous drainage member having flow control members which project above the surface of the drainage member a distance less than the thickness of the fibrous structure formed thereon and the fibrous structure may be later densified in a hard nip. Another teaching that fiber concentrations in areas of a fibrous structure may be dispersed so that, dependent upon the length of the fibers, island areas of extremely thin cross-section may be produced is shown by Osborne U.S. Pat. No. 3,322,617 issued May 30, 1967.
Finally, several attempts to provide an improved foraminous member for making such cellulosic fibrous structures are known, one of the most significant being illustrated in Johnson et al. U.S. Pat. No. 4,514,345 issued Apr. 30, 1985. Johnson et al. teaches hexagonal elements attached to the framework in a batch liquid coating process.
However, one problem present with the paper made according to each of these references is that the tensile strength of such paper is limited by the strength of the high basis weight regions of such paper. If the high basis weight regions are strengthened by adding more fibers, a noneconomical use of raw materials results.
Another problem with the paper made according to the foregoing references is that the absorbency is limited by the low basis weight regions of the paper. Because the low basis weight regions are taught to be of constant density and thickness, such paper is limited in how absorbent it will be for the user.
One explanation for the limited properties of the paper produced according to the prior art may be that such paper is produced entirely in registration with the protuberances, as taught in the aforementioned references. That is, after the fibrous slurry which forms the paper having plural basis weights is deposited on the Fourdrinier wire, all subsequent operations, such as drying, etc., are carried out in registration with the high and low basis weight regions as originally formed.
One attempt to vary the density of paper made according to the prior art is by joining two plies of the paper together and knob-to-knob embossing the resulting laminate as taught in Wells U.S. Pat. No. 3,414,459 issued Dec. 3, 1968. However, while this operation increases the density of the embossed areas, it has no effect on basis weight and adds a converting step to the papermaking process.
Accordingly, it is an object of this invention to overcome such problems of the prior art and particularly to overcome such problems as they relate to a single lamina of paper. Specifically, it is an object of this invention to provide a paper which increases the tensile strength through providing a stronger high basis weight region, without substantially increasing the number of fibers utilized to make the high basis weight region. Also, it is an object of this invention to provide low basis weight regions having enhanced absorbency by providing plural densities and/or plural projected average pore sizes in such low basis weight regions. Further, it is an object of this invention to provide plural densities and/or plural projected average pore sizes without a dedicated converting operation, such as embossing. It is also an object of this invention to accomplish the foregoing without radical departure from known papermaking machinery and techniques.
The foregoing may be accomplished by carrying out steps in the process of forming the claimed cellulosic fibrous structure which comprise operations which are selectively applied to regions of the fibrous structure, which selected regions are not coincident the regions distinguished and defined by mutually different basis weights or densities. Particularly, the step of applying a noncoincident differential pressure to the fibrous structure is useful. Such noncoincidence may occur through differences in size, pattern registration, or combinations thereof, between the originally formed plural basis weight and density regions and the regions to which a differential pressure is selectively applied.